Line pipes for use in crude oil or natural gas producing oil wells must have mechanical properties to withstand high temperatures and high pressures, and excellent corrosion resistance. With an increase in the depth of oil wells in recent years, such pipes are subjected to higher temperatures and higher pressures, while with sites of production shifted from the land to the sea, these pipes are inevitably used in oil wells containing chlorine ion, hydrogen sulfide and carbon dioxide. Thus, improved mechanical properties and higher corrosion resistance are required of such pipes.
Since pipes of single layer are unable to have both of these characteristics, pipes of two-layer structure have been proposed which comprise an outer layer of carbon steel or low alloy steel, and an inner layer of stainless steel, high alloy steel or like corrosion resistant alloy.
Pipes for the above use are elongated pipes of small diameter generally measuring about 100 to about 600 mm in outside diameter, about 4000 to 6000 mm in length, about 10 to about 60 mm in outer layer thickness and about 2 to 4 mm in inner layer thickness. Conventional two-layer pipes for use as line pipes in oil wells or the like are produced by preparing an inner-layer pipe and an outer-layer pipe first, fitting the two pipes together and subjecting the pipe assembly to a hydraulic press, hot rolling, hot extrusion or explosive pressure-bonding process.
These two-layer pipes have problems. Firstly, the pipes have increased eccentricity. Secondly, some of the pipes have insufficient bond strength because the outer layer and the inner layer are not fully in metal-to-metal intimate bonding contact with each other but are mechanically joined together locally. Moreover, since the composition of the material forming the pipe distinctly differs at the boundary between the outer layer and the inner layer, the pipe has reduced strength in its entirety. Further when the pipe is used in a sour environment, hydrogen sulfide penetrates into the mechanically joined portion during use, inducing at the joint.
If the two-layer pipe is produced in the form of a composite pipe by centrifugal casting, the outer layer and the inner layer can be metallurgically joined together with enhanced bond strength, whereas another problem will then arise. The material of the inner layer and the material of the outer layer diffuse into each other, impairing the mechanical strength of the outer layer and the corrosion resistance of the inner layer.
We have found that such an elongated pipe of small diameter can be produced by centrifugal casting in the form of a composite pipe having satisfactory bond strength without permitting diffusion of the materials of the outer and inner layers into each other when the pipe is so cast that an intermediate layer of a predetermined thickness is formed between the outer layer and the inner layer. The pipe can be produced as a metallugically integral pipe without permitting the materials of the outer and inner layers to mix with each other except at the intermediate layer, so that the outer layer exhibits the contemplated mechanical properties, with the inner layer retaining the specified corrosion resistance.